Crankshafts for internal combustion engines must have their crank pins finished to a high degree of precision. Because of the peculiar shape and geometry of the crankshaft, machining of the crank pins to their finished diameter is a complex operation, and many specialized machines have been devised for this purpose. While this art has been extensively developed, the nature of the machining operation and the paramount requirement for an accurately and precisely finished product frequently require design compromises which achieve accuracy at the expense of production time or tool life.
One type of crankshaft pin milling machine in present-day use employs an internal milling cutter. The cutter is mounted for rotation and for vertical reciprocation in a cutter carrier which may be axially positioned on the ways of a machine in which the crankshaft to be finished is chucked within the crankshaft cutter. The cutter is axially positioned with its teeth aligned with a crank pin to be finished and is then moved vertically, with the cutter being driven in rotation, until the teeth contact and machine a point on the periphery of the pin to its desired final diameter. The crankshaft is then rotated slowly about its main axis while at the same time the cutter is moved vertically to maintain the cutter teeth in tangential contact with the pin periphery at a fixed radial distance from the pin axis corresponding to the desired finished radius of the pin. Upon completion of one revolution of the crankshaft about its main axis, the pin surface has been milled to the desired finished radius.
For reasons which will be discussed more fully below, the angular increment of pin surface machined by the foregoing technique does not have a constant or linear relationship to a given angular increment of rotation of the crankshaft about its main axis. In fact, depending upon the cutter diameter and the dimensions of the crankshaft being machined, the geometry of the system is such that the amount of metal machined from the pin during a ten degree advance of the crankshaft in rotation about its main axis at one point in its rotary cycle may be three or more times as great as the amount of metal machined from the pin during a ten degree rotation of the crankshaft at some other point in its cycle of revolution.
While the foregoing problem has been recognized in the industry, the prior art does not disclose any specific practical solution. In an effort to achieve a maximum production rate, the crankshaft is driven at a rate of rotation such that, during those portions of the cycle where the maximum amount of metal is being removed, the cutter teeth are overloaded with a consequent reduction in tool life.